1. Field of the Invention
The present invention relates to novel synthetic precipitated silicas and to a process for producing a synthetic precipitated silicon dioxide having a new and improved combination of properties and characteristics. More particularly, the invention relates to the production of precipitated high structure silicas produced by reacting aqueous alkali metal silicate solutions with an acidification agent. The novel products are high structure finely divided silicas having certain unique properties particularly with respect to structure index, oil absorption, void volume, surface activity, friability, wetting characteristics, and further properties. The novel products are especially suitable for use as reinforcing agents in rubber, in paints, paper, detergents, molecular sieves, and in polymeric compositions.
2. Description of the Prior Art
As known in the art, finely divided silica or silicon dioxide particulates can be prepared by the acidulation of an aqueous silicate solution with an acid, such as sulfuric acid. Such products are commercially available and are characterized by, and have, the following properties: high structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area, and low pack density. Because of properties such as high oil absorption, the pigments have been successfully used as reinforcing pigments in rubber. However, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are decreased. Further, the aforementioned properties of said known and commercially available silicas render them unsuitable for many uses.
In this regard and generally speaking for the moment, known processes including those described in the literature as well as those techniques employed in the industry produce products suitable for use as reinforcing fillers. Thus, in U.S. Pat. No. 2,940,830 which issued June 14, 1960 to F. S. Thornhill, there is described a process for preparing finely divided silicas which are suitable as reinforcing agents in rubber compositions. Thornhill more specifically describes a process of preparing a silica material which is characterized by having an average ultimate particle size of 0.015 to 0.04 micron and a surface area of 25 to 200 square meters per gram by the controlled rate of addition of acid to an alkali metal silicate wherein the resultant slurry is constantly maintained at a pH above 7 in order to achieve the aforementioned end product characteristics. The Thornhill patent is specifically directed to the production of a product suitable as a reinforcing agent in rubber compositions.
In U.S. Pat. No. 3,235,331, which issued Feb. 15, 1966 to Nauroth et al, there is described a process for producing a precipitated silica which is also stated to be useful as a reinforcing agent for rubber. More specifically, this patent discloses a process wherein an aqueous alkali metal silicate solution and acid are simultaneously added to a reaction vessel. In the Nauroth et al patent, it is pointed out that this simultaneous addition is continued until the viscosity of the pool rises through a maximum and then falls to a substantially lower value. The amount of the acidification agent and the alkali metal silicate are proportioned so as to maintain the pH of the resulting slurry substantially constant throughout the major portion of the reaction and in the range of about 10 to 12. The process is generally conducted at a temperature of 80.degree.0 to 90.degree. C. and the end product, after drying, results in a silica which may have a surface area of 260 square meters per gram. The patentees point out that the product is satisfactory as a reinforcing agent for rubber.
In U.S. Pat. No. 3,445,189 issued May 20, 1969 to Maat et al, there is described a process for producing finely divided silicic acid by simultaneously adding solutions of an alkali silicate and a strong mineral acid to water at a temperature between 70.degree. C. and 90.degree. C. while maintaining the reaction pH between 7 and 9. The patentees point out that the product obtained by the aforementioned process is a finely divided nongelatinous silicic acid which is useful as a filler for natural and synthetic rubber and other elastomers. It is also disclosed in this patent that for a silica to be useful as a filler for natural and synthetic rubber and other elastomers, its surface area and oil absorption are of vital importance. This patent further discloses that extensive investigations have further indicated that if a finely divided silicic acid is to have good reinforcing properties for rubber, it must have a surface area of 100 to 250 m.sup.2 /g and an oil absorption of more than 2 cc/g or 200 cc/100 g. See column 2, lines 18 through 22.
In U.S. Pat. No. 3,730,749, which issued May 1, 1973 to James E. Morgan, there is diclosed a process for preparing silica for use in reinforcing compositions. It is pointed out in Morgan that the viscosity increase which occurs during the acidification or neutralization of aqueous alkali metal silicate is substantially minimized by adding a controlled amount of an alkali metal silicate. In Examples I, II, and III of this patent, it is also noted that the silica filter cakes had solid contents of 18.5; 24.9; and 25.1 percent, respectively. This means that the percent wet cake moisture of the silicas disclosed in Examples I, II, and III is one hundred minus the percent solid content in the filter cake. In other words, the percent wet cake moisture (% WCM) of silicas mentioned in Examples I, II, and III is 81.5; 75.1; and 74.9, respectively. The surface area, the average ultimate particle sizes, and rubber data of silicas produced by the teachings of Examples II and III are listed in Table 3 which also sets forth that rubber compositions incorporating the silicas of Examples II and III have desirable rubber properties. It is further substantiated by this patent that rubber properties of silicas are related to the high wet cake moisture of the silica pigment. Thus, it is taught that a silica of high percent wet cake moisture and suitable particle size and surface area has better rubber properties than the corresponding material of lower wet cake moisture. Thus, the silicas disclosed in Morgan have a higher structure index, and therefore the silicas are useful rubber reinforcing fillers.
From the above it will be seen that the structure index of a silica is related to the rubber properties-- a silica of higher structure index will have better rubber properties than a silica of lower structure index. At this point and before turning to the remarkable concept of the present invention, the various types of synthetic silicas, as well as "structure" and "structure index" should therefore be discussed.
In this regard, and as known and accepted in the art, commercially available synthetic silicas are derived either by a liquid phase or a vapor process. Silicas obtained by the vapor process are called fumed or pyrogenic silicas. Products obtained by the liquid process are categorized as silica gels and precipitated silicas. Thus, there are three distinct types of synthetic silicas on the market: